1. Field of the Invention
The present invention relates generally to a video tape recorder having a recording mode discriminating circuit and more particularly, is directed to a recording mode discriminating circuit for use with a video tape recorder (VTR) having a plurality of recording modes.
2. Description of the Prior Art
Conventionally, a video tape recorder has a number of recording systems corresponding to a normal mode and a high image quality mode. In the high image quality mode, a so-called high band system is employed, in which a carrier frequency of an FM (frequency modulated) luminance signal is shifted to a frequency higher than that of the normal mode and a frequency deviation range is widened, in order to obtain a reproduced picture of high image quality.
In the case of 8-mm type video tape recorder, FM luminance signals YFM of respective modes shown on the following table 1 are recorded together with a down converted chroma signal CL, an audio signal AFM, etc., on a magnetic tape in the frequency allocations shown in FIGS. 1A and 1B.
TABLE 1 ______________________________________ High image quality mode normal mode ______________________________________ luminance signal FM modulation FM modulation recording system recording recording white peak frequency 7.7 MHz 5.4 MHz sync. chip frequency 5.7 MHz 4.2 MHz frequency deviation 2.0 MHz 1.2 MHz pre-emphasis time 0.47 .mu.s 1.3 .mu.s constant clipping level white clip 220% 220% dark clip 90% 90% ______________________________________
In the reproducing system 1, a recording mode discriminating circuit is utilized in order to automatically discriminate a plurality of the above-mentioned recording modes and to reproduce a picture.
A conventional video tape recorder capable of coping with a plurality of recording modes is arranged as, for example, shown in FIG. 2.
Referring to FIG. 2, there is provided a luminance signal reproducing system 10. A reproduced RF (radio frequency) signal from a magnetic head 1 is commonly supplied through a playback amplifier 2 to a high-pass filter 3 and a low-pass filter 4 which are used to perform a so-called Y/C separation. A down converted chroma signal CL from the low-pass filter 4 is supplied to a color signal processing system 5 and an FM luminance signal YFM from the high-pass filter 3 is supplied to an input terminal 10i of the luminance signal reproducing system 10.
The FM luminance signal YFM is commonly supplied to RF band processing circuits 11h and 11n of the high image quality mode and of the normal mode, respectively. Limiters 12h and 12n for preventing inversion and band-pass filters 13h and 13n are connected in cascade to the band processing circuits 11h and 11n, respectively whereby, together with the magnetic head 1, a proper band-processing such as a peaking-process or the like is performed so as to balance double side-band waves of the FM signal. The limiters 12h and 12n are each constructed as a soft limiter or a double-limiter type, and the outputs of the band-pass filters 13h and 13n are supplied through a change-over switch S1 to an FM demodulator 14.
An output of the FM demodulator 14 is supplied through low-pass filters 15h and 15n of high image quality mode and normal mode to de-emphasizing circuits 16h and 16n. The frequency characteristics of the low-pass filters 15h and 15n are set, for example, to 5 MHz flat and 3 MHz flat in response to the frequency allocations shown in FIGS. 1A and 1B to thereby provide horizontal resolutions of about 430 horizontal scanning lines and about 270 horizontal scanning lines, respectively. Reproduced luminance signals Y from the de-emphasizing circuits 16h and 16n are supplied through noise reducing circuits 17h and 17n including comb-filters and a change-over switch S2 to an output terminal 10o.
Referring to FIG. 2, there is provided a recording mode discriminating circuit 20, in which the FM luminance signal YFM from the high-pass filter 3 is supplied through a limiter 6 to an input terminal 20i thereof. Band-pass filters 21h and 21n for high image quality mode and normal mode are commonly connected to the input terminal 20i. Central frequencies of the two band-pass filters 21h and 21n are set to sync. chip frequencies fsh and fsn of the high image quality mode and the normal mode as shown on the earlier noted table 1 where fsh is 5.7 MHz and fsn is 4.2 MHz, respectively.
Detecting circuits 22h, 22n and hold circuits 23h, 23n are connected in cascade to the two band-pass filters 21h and 21n, respectively. Outputs of the hold circuits 23h and 23n are supplied to a comparator 24 and an output of this comparator 24 is supplied through a terminal 20o to the change-over switches S1 and S2.
When the FM luminance signal YFM supplied to the recording mode discriminating circuit 20 is the signal of the high image quality mode, the output of the hold circuit 23h generally becomes larger than that of the hold circuit 23n, thus making the output of the comparator 24 "high" level.
Conversely, when the FM luminance signal YFM is the signal of the normal mode, the output of the hold circuit 23n generally becomes larger than that of the hold circuit 23h, thus making the output of the comparator 24 "low" level.
By the above-mentioned discriminated outputs, the switches S1 and S2 are connected to the predetermined positions in response to the recording modes.
Incidentally, the frequency and level of the luminance signal are considerably changed in association with the pattern of the original picture, and the frequency spectrum of the FM luminance signal YFM is also changed in association therewith.
As a consequence, as, for example, shown in FIGS. 1A and 1B, even in the signal of the high image quality mode in which a carrier frequency corresponding to white level of 50% satisfies f50h=7.0 MHz as shown in FIG. 1B, if a frequency fy of an original luminance signal satisfies the following equation (1), a frequency of an m-order lower side band wave falls in the vicinity of the sync. chip frequency fsn of the normal mode as shown in FIG. 3. In FIG. 3, m=2 is established. EQU m.multidot.fy.apprxeq.f50h-fsn (1)
In that event, in the mode discriminating circuit 20 shown in FIG. 2, there is then the risk that the reproduced signal of the high image quality mode shown in FIG. 3 is erroneously identified as the reproduced signal of the normal mode.
To solve this problem of erroneous discrimination, the assignee of the present application has previously proposed a mode discriminating circuit in which, when a signal component exists near the sync. chip frequency fsn of the normal mode, it is determined on the basis of the existence or absence of the m-order upper side band wave whether the reproduced signal is the reproduced signal of the high image quality mode or not, thereby preventing an erroneous discrimination (see Japanese Patent Application No. 63-75517 or Japanese Laid-Open Patent Gazette No. 1-246975). FIG. 4 shows such previously-proposed mode discriminating circuit 20A. In FIG. 4, like parts corresponding to those of FIG. 2 are marked with the same references and therefore need not be described in detail.
As shown in FIG. 4, in this mode discriminating circuit 20A, a central frequency fuh of the bandpass filter 21A of the high image quality mode is set as expressed by the following equation. EQU fuh=f50h+m.multidot.fy.apprxeq.2.multidot.f50h-fsn (2)
Accordingly, with respect to the carrier frequency f50h of the high image quality mode signal corresponding to the white level of 50%, this frequency fuh becomes symmetrical to the sync. chip frequency fsn of the normal mode and is determined in the example of the aforenoted numeric values as follows. EQU fuh=9.8 MHz
Further, the pass band width of the bandpass filter 21A corresponds to the change of the pattern of the original picture and is therefore determined to be relatively wide. The rest of FIG. 4 is similar to the mode discriminating circuit 20 of FIG. 2.
As shown in FIG. 3, in the FM luminance signal of the high image quality mode, when the m-order lower side band wave exists in the vicinity of the sync. chip frequency fsn of the normal mode, an m-order upper side band wave exists symmetrically about the carrier wave.
Accordingly, in the mode discriminating circuit 20A of FIG. 4, the recording mode is discriminated as in the following table 2.
TABLE 2 ______________________________________ fsn component exists exists none none fuh component exists none exists none discrimination High image normal High image High image mode quality mode quality quality mode mode mode ______________________________________
In the mode discriminating circuit 20A of FIG. 4, however, an electrolytic capacitor of relatively large capacitance, for example, about 10 .mu.F is utilized in each of the hold circuits 23h and 23n in order to reduce noise. There is then the disadvantage that the mode discriminating circuit 20A of FIG. 4 cannot be fabricated as an IC (integrated circuit) without difficulty.
Further, load resistances of the respective detecting circuits 22h and 22n are set as relatively large values, for example, about 10 k.OMEGA. in order to suppress the consumption of current so that a discharge time constant becomes large, which urges the discrimination of recording mode to be performed at low speed.
Incidentally, in the case of an 8-mm video tape recorder, fundamentally, a coating type metal (MP) tape is utilized in the normal mode while an evaporation type metal (ME) tape is utilized in the high image quality mode. Further, it is frequently observed that a high efficiency coating metal (MPHG) tape is utilized in the high image quality mode.
The conventional 8-mm video tape recorder capable of coping with a plurality of modes such as the normal mode and the high image quality mode is arranged as, for example, shown in FIGS. 5A and 5B.
Referring to FIG. 5A, there is shown a luminance signal recording system in which reference numeral a luminance signal Y applied to an input terminal 30i is commonly supplied to a high image quality mode band limiting circuit 31h and a normal mode band limiting circuit 31n, thereby preventing an aliasing noise in the spectrum of an FM signal. Outputs of the band limiting circuits 31h and 31n are supplied through pre-emphasizing circuits 32h and 32n to modulation setting circuits 33h and 33n, in which DC levels and AC amplitudes thereof are set to predetermined values of the two modes. Outputs of the modulation setting circuits 33h and 33n are supplied through the change-over switch S1 to an FM modulator 34, whereby the FM signal YFM on the preceding table 1 is output from the FM modulator 34. This output signal YFM is supplied through equalizing circuits 35h and 35n and the change-over switch S2 to an output terminal 30o. In an adder 100, the FM luminance signal YFM and the down converted chroma signal CL from a recording-side color signal processing circuit 102 are added and then fed through a recording amplifier 103 to a magnetic head Hh for high image quality mode.
Referring to FIG. 5B, there is provided a luminance signal reproducing system, in which a reproduced RF signal from a magnetic head Hh is through a playback amplifier 104 to a Y/C separating circuit 105. The thus separated chroma signal CL is supplied to a reproducing-side color signal processing circuit 106, and the FM luminance signal YFM is supplied through an input terminal 40i to high image quality and normal mode RF band processing circuits 41h, respectively and 41n.
In the band processing circuits 41h and 41n, the band processing such as the peaking of frequency characteristics shown in FIGS. 6A and 6B or the like is performed so as to balance double side band waves of the FM signal in the form of being applied to the magnetic head Hh and succeeding inversion preventing circuits 42h and 42n. The inversion preventing circuits 42h and 42n each include soft limiters and bandpass filters though not shown, and outputs of the inversion preventing circuits 42h and 42n are supplied through a change-over switch S3 to an FM demodulator 43.
An output of the FM demodulator 43 is supplied through low-pass filters 44h and 44n of high image quality mode and normal mode to de-emphasizing circuits 45h and 45n. The frequency characteristics of the low-pass filters 44h and 44n are set to, for example, 5 MHz flat and 3 MHz flat to thereby provide, for example, horizontal resolutions of 400 and 250 horizontal scanning lines and a predetermined S/N (signal-to-noise ratio) ratio. Reproduced luminance signals Y from the de-emphasizing circuits 45h and 45n are supplied through noise reducing circuits 46h and 46n including comb filters and a change-over switch S4 to an output terminal 40o.
In the video tape recorder capable of coping with only the normal mode recording system, however, not only the recording in the high image quality mode recording system is unsatisfactory but also the reproduction of the high image quality mode cannot be performed because a satisfactory reproduced signal cannot be obtained due to the frequency characteristic shown in FIG. 6B.
Further, in the conventional video tape recorder capable of coping with both the normal mode and the high image quality mode, most of the luminance signal system circuits of the recording and reproducing sides are provided exclusively for their own modes and generally formed of two systems. Therefore, the circuit arrangement is large as compared with the video tape recorder capable of coping with only the normal mode and the manufacturing cost thereof is unavoidably increased.